1. Field of the Invention
This invention relates generally to lateral injection vertical cavity surface-emitting lasers (VCSELs) and, more particularly, to such lasers that provide for both improved current distribution and reduced parasitic capacitance.
2. Background of the Invention
A conventional lateral injection VCSEL as depicted in FIG. 1 comprises an active region 60 disposed in a cavity resonator formed by a pair of multi-layered mirrors 52 and 54. A current confinement structure 58 constrains current to flow in a relatively narrow channel through the active region. The structure 58 includes a high resistivity layer 57 having an aperture 59 through which current flows from an annular upper contact 54 to an annular lower contact 56. More specifically, as shown by the dashed lines 40 and 42, current flows from upper contact 54 into high conductivity layer 46, continues essentially horizontally along layer 46, then flows vertically through aperture 59 to high conductivity current return layer 62, and continues essentially horizontally along layer 62 to contact 56. In this symmetrical electrode configuration, the current flow paths 40 (i.e., those nearer to the edge of aperture 59) are shorter than paths 42 (i.e., those nearer to the center of aperture 59). Consequently, the current density is higher near the edges of the aperture than in the central portion of the aperture. This type of non-uniform current distribution favors higher order transverse optical modes, which have higher intensity near the edges of the aperture, over the fundamental transverse mode, which has its peak intensity in the center of the aperture. However, operating a VCSEL in the fundamental mode is generally preferred since this mode can be more efficiently coupled into other devices, optical fibers in particular.
This problem is recognized in the prior art as described by M. P. Creusen et al., Netherlands Patent No. 1005570 (hereinafter, Creusen) laid open to public inspection on Sep. 22, 1998. In order to make the current density distribution in the aperture more nearly uniform, Creusen proposes an asymmetric contact structure as shown in FIG. 1 of the patent. Upper contact (electrode) 13 is formed on upper contact layer 6 on one side of the mirror 1, and lower contact (electrode) 14 is formed on contact layer 7 on the laterally opposite side of the mirror 1. Creusen concludes that because the probability of all current paths through the active layer 5 is now the same, the homogeneity of the current injection will actually increase. As a result the higher order transverse modes is (sic) suppressed. This conclusion is flawed since it focuses on only the direct current paths through the aperture and neglects parallel paths through which the current flows. These parallel paths, where the current path may balloon in a longer, arc-like trajectory, do not all have the same length and, therefore, give rise to nonuniformity in the current density distribution in the aperture. These parallel paths exist because the top contact layer 6 and bottom contact layer 7 are not quite equipotentials, even if they are designed with a multiplicity of high and low doped layers in order to minimize the voltage drop across the device from upper contact 13 to lower contact 14. That Creusen uses this multiplicity of high and low doped layers to form the contact layers is clear from the cross-reference therein to the Jewell et al. U.S. Pat. No. 5,245,622, which is incorporated herein by reference.
Thus, a need remains in the art for a lateral injection VCSEL design that provides an improved current distribution in the aperture that favors the fundamental transverse mode over higher order modes.
In addition, the speed of operation of prior art lateral injection VCSELs, including those of Creusen, is limited by parasitic capacitance. The upper metal contact and the high conductivity semiconductor upper contact layer form one plate of the parasitic capacitor, and the lower metal contact and the high conductivity semiconductor lower contact layer form the other plate. The remaining semiconductor material in between (e.g., the high resistivity aperture layer, the active region) forms the dielectric of the capacitor. In lateral injection VCSELs the plates are typically closer together than in conventional (i.e., through-the-mirror injection) VCSELs, making the parasitic capacitance per unit area larger. The higher capacitance increases the RC time constant of these lasers and, therefore, increases their rise and fall time when they are turned on and off, which in turn limits their modulation speed.
Thus, a need remains in the art for a lateral injection VCSEL design that reduces parasitic capacitance and thereby improves its speed of operation.
In accordance with one aspect of our invention, a lateral injection VCSEL comprises upper and lower mirrors forming a cavity resonator, an active region disposed in the resonator, high conductivity upper and lower contact layers located on opposite sides of the active region, upper and lower electrodes disposed on the upper and lower contact layers, respectively, and on opposite sides of the upper mirror, and a current guide structure including an apertured high resistivity layer for constraining current to flow in a relatively narrow channel through the active region, characterized in that a portion of the lower contact layer that extends under the top electrode has relatively high resistivity. This feature of our invention serves two purposes. First, it suppresses current flow in parallel paths and, therefore, tends to make the current density distribution in the aperture more favorable for the fundamental mode. Second, it reduces parasitic capacitance.
In a preferred embodiment, the upper electrode is U-shaped (top view) with the mouth of the U facing the lower electrode. Likewise the high resistivity portion of the lower contact layer is also U-shaped and similarly oriented. Both surround a significant portion of the aperture and provide an opening that faces the lower electrode. Together, the U-shaped upper electrode and the U-shaped high resistivity portion of the lower contact layer define a high conductivity direct current path from the upper electrode through the aperture to the lower electrode.
In another preferred embodiment the high resistivity portion does not extend to the interior edges of the U-shaped upper contact; that is, the resistivity is patterned in such a way that a band or corridor of high conductivity is retained along the interior edges of the U-shaped electrode. This VCSEL design reduces parasitic capacitance by approximately an order of magnitude and prevents any significant increase in series resistance.
In yet another embodiment of our invention, the structure in which the upper and lower contact layers are formed is asymmetric with respect to the resonator axis, being wider on the side of the upper mirror where the upper electrode is located and narrower on the opposite side of the upper mirror. This feature further reduces parasitic capacitance.